Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the ion beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
Solar cells are one example of a device that uses silicon workpieces. Any reduced cost to the production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of a clean energy technology.
Solar cells typically consist of a p-n semiconducting junction. FIG. 1 is a cross-sectional view of an interdigitated back contact (IBC) solar cell. In the IBC solar cell 205, the junction is on the back or non-illuminated surface. In this particular embodiment, the IBC solar cell 205 has an n-type base 206, an n+ front surface field 207, a passivating layer 208, and an anti-reflective coating (ARC) 209. The passivating layer 208 may be SiO2 and the ARC 209 may be SiNx in one instance, though other materials or dielectrics may be used. Photons 214 enter the IBC solar cell 205 through the top (or illuminated) surface, as signified by the arrows. These photons 214 pass through the ARC 209, which is designed to minimize the number of photons 214 that are reflected away from the IBC solar cell 205. The photons 214 enter through the n+ front surface field 207. The photons 214 with sufficient energy (above the bandgap of the semiconductor) are able to promote an electron within the valence band of the semiconductor material to the conduction band. Associated with this free electron is a corresponding positively charged hole in the valence band.
On the back side of the IBC solar cell 205 is an emitter region 215. The doping pattern of the emitter region 215 is alternating p-type and n-type dopant regions in this particular embodiment. The n+ back surface field 204 may be approximately 450 μm in width and doped with phosphorus or other n-type dopants. The p+ emitter 203 may be approximately 1450 μm in width and doped with boron or other p-type dopants. This doping may enable the junction in the IBC solar cell 205 to function or have increased efficiency. This IBC solar cell 205 also includes a passivating layer 212, n-type contact fingers 210, p-type contact fingers 211, and contact holes 213 through the passivating layer 212.
To form the IBC solar cell 205, at least two patterned doping steps may be required. If the p+ emitter 203 and n+ back surface field 204 overlap after these patterned doping steps and the overlap region has high dopant concentrations for both n-type and p-type dopants, there will be a very narrow depletion region between the p+ emitter 203 and n+ back surface field 204. This means that shunting between the p+ emitter 203 and n+ back surface field 204 can occur. High dopant concentrations between 1E18 cm2 to 1E19 cm−2 or around the mid E19 cm−2 may lead to shunting.
To avoid such shunting, tight alignment of the p+ emitter 203 and n+ back surface field 204 is required so that no such overlap occurs. However, even if no overlap occurs, if the p+ emitter 203 and the n+ back surface field 204 touch then charge carriers can cross between the p+ emitter 203 and the n+ back surface field 204 using quantum tunneling. In such a case, the space charge region between the emitter 203 and the n+ back surface field 204 will be shallow, enabling the quantum tunneling. Since the p+ emitter 203 and the n+ back surface field 204 are located on the same side of the IBC solar cell 205 in the emitter region 215, such quantum tunneling also can shunt any junction between the p+ emitter 203 and the n+ back surface field 204.
The IBC solar cell 205 may be improved by increasing the distance between the p+ emitter 203 and the n+ back surface field 204 to, for example, approximately 1 μm. However, maintaining alignment of the two patterned doping steps to keep such a distance while ensuring the p+ emitter 203 and the n+ back surface field 204 are not close enough for quantum tunneling is difficult. Increasing this distance between the p+ emitter 203 and the n+ back surface field 204 to larger dimensions also has problems. If large undoped regions exist between the p+ emitter 203 and the n+ back surface field 204, then charge carriers will not be repelled from the surface of the IBC solar cell 205. Unless this surface is well-passivated, recombination can occur at the surface of this undoped region. Recombination degrades voltage and current of the IBC solar cell 205. Therefore, there is a need in the art for an improved method of doping solar cells and, more particularly, an improved method of doping IBC solar cells using ion implantation.